Time to put on the ol' speculation hat.

Purely from a mechanics standpoint, I'm honestly imagining Ninja to be a tank. SE is already desperately trying to push people into playing tanks by offering mounts and bonus gil, and there's still somewhat of a lack. If they were playing their cards right, they'd make Ninja a tank as a means to coerce more people into playing the role.

That being said, I could see them playing it off by giving them a sort of passive HP buff they can flip on, like Warrior. Maybe call it Shadow Clone, and have it increase their HP and evasion by a certain percentage, as well as enmity generation. Various "ninjitsu" abilities to increase defense and evasion, etc. Ultimately, it would give FFXIV the three staple tank archetypes. Sword and board, two-hander, and evasion. I feel like them adding two more DPS classes into an already DPS heavy game would be a mistake they're smart enough not to make. But, who knows? Only SE at this point~

I could imagine Ninjas as having been sort of the "protectors" of the Domans, which Yugiri is a shining example of. Using their superior agility and ninjitsu to distract and disable enemies and the like. In line with that, thief would be much more of your stereotypical damage-dealing dagger wielder. Sneak attacks, backstabs, dirty fighting, all that good stuff that you might imagine a shadowy Lominsan organization might employ.